1. Field of the Invention
The present invention relates to a fuel injection control system for an engine and, in particular, to a fuel injection control system which injects fuel a plurality of times to each cylinder during each cycle.
2. Description of Related Art
A conventional engine fuel injection control system is disclosed in Japanese Utility Model Laid-Open No. 61-76,143. This conventional control system provides two fuel injection time points during each cycle of each cylinder of the engine. More specifically, it provides a first injection time point on the leading side and a second injection time point on the trailing side and controls the fuel injection ratio between the first and second injection time points based on the engine load and intake air temperature. As a result, vaporization of the injected fuel is promoted and combustion is stabilized even when the engine load and the intake air temperature vary.
On the other hand, there is known another conventional engine in which fuel consumption is reduced during low engine load by causing a rich air fuel mixture to be present near the ignition plug and a lean air fuel mixture to be present below the rich mixture. This will be referred to as "stratified combustion" in this specification. In an engine adopting stratified combustion, a control system sets a trailing side injection time point which is favorable for the stratified combustion, and a leading side injection time point which is favorable for achieving uniform combustion during high engine load, and controls the fuel injection ratio between the trailing side injection time point and the leading side injection time point based on driving conditions such as the engine load and speed.
The above-mentioned two types of conventional fuel injection control system have to calculate the fuel injection ratio or injection division between the leading side injection time point and the trailing side injection time point based on maps corresponding to the various driving conditions. These system types further have to calculate the injection amounts at the leading side injection time point and the trailing side injection time point based on the above-mentioned ratio in every cycle. As a result, the ratio and injection amount calculations become very complicated, and fuel supply is liable to become less than that required during acceleration and other transient engine operation conditions owing to delays in calculating the injection ratio.